CHICAGO —
The U.S. introduction of a vaccine to prevent cervical cancer in 2006 has reduced infections with the human papillomavirus or HPV - the sexually-transmitted virus that causes the disease - by more than half among girls and young women, U.S. health officials said on Wednesday.

The results were better than expected and may even suggest that unvaccinated individuals are benefiting because of a drop in the number of infections circulating, the team reported in the Journal of Infectious Diseases.

“This report shows that HPV works well, and the report should be a wake-up call to our nation to protect the next generation by increasing HPV vaccination rates,” Dr Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, said in a statement.

Frieden said only a third of U.S. girls aged 13-17 have been fully vaccinated with HPV vaccines, which include Merck's Gardasil and GlaxoSmithKline's Cervarix.

That compares with far higher vaccination rates in other countries such as Rwanda, where more than 80 percent of teenage girls have been vaccinated.

In the study, a team led by the CDC's Dr. Lauri Markowitz used data from a large health survey to compare rates of infection with certain strains of the HPV virus among girls and women aged 14-19 in the four-year period before the introduction of the vaccine (2003-2006) and after its introduction (2007-2010).

The study largely reflects the impact of Merck's Gardasil. In 2006 it became the first HPV vaccine to win U.S. approval.

Gardasil protects against four HPV strains known to cause cervical cancer and genital warts. Glaxo's Cervarix won U.S. approval in 2009 and protects against two of the most common cancer-causing strains of HPV.

The researchers found the vaccine worked even better than expected, reducing by 56 percent the number of infections caused by strains of HPV covered by the vaccine among women and girls aged 14-19.

Markowitz said the higher than expected response rate could be the result of so-called “herd immunity,” in which the vaccine is also reducing infections among those who are not vaccinated. Or it could mean that the vaccine was working even among women who had not received the full three doses, which included about 49 percent of women in the study.

The CDC recommends routine HPV vaccination for boys and girls at age 11-12. But only about half of all U.S. girls have gotten at least one of the three recommended shots, and far fewer boys have gotten the first dose of the vaccine.

According to the CDC, HPV infections cause about 19,000 cancers each year among women in the United States, of which cervical cancer is the most common. HPV infections also cause about 8,000 cancers a year in U.S. men, of which throat cancers are the most common.